The Sleep Diet

Snooze to lose ... adequate sleep is important for maintaining a healthy weight.

If you have problems getting to sleep you'll know all the good zzz rules by heart. Go to bed and wake up at the same time each day, keep the bedroom off limits to all activities except sleep and sex – and avoid sleep thieves like caffeine, nicotine and alcohol.But what if you're sticking to the rules and still not getting your eight hours? One possibility is a too high body temperature – even in the middle of winter, says Sydney sleep scientist Carmel Harrington.

"It's easier to get to sleep on a decreasing temperature. Normally the body's temperature is at its highest at around 7pm after which it starts to fall, getting you ready for sleep," she explains. "But some things - including exercising and eating late – can raise your core temperature, making it difficult to get to sleep or stay asleep."This isn't an argument to skip exercise or eating, but if sleep is fragile it's worth trying to do both earlier in the evening rather than later, she says.

In the case of exercise which also raises levels of cortisol, a stress hormone that helps us stay alert, it's smart to leave at least three hours between workout and lights out in order to give your body time to cool down. The same goes for eating. Along with the temperature issue, there's the fact that digestion slows down at night so that a meal that normally takes around two to three hours to digest takes longer at night and the discomfort can disrupt your sleep, Harrington says.

Other ways to stay cool enough for sleep include keeping the bedroom at the ideal sleep temperature of around 18 degrees, and having a warm, not hot, shower before bedtime – the effect of your skin cooling down after the shower tricks your body into thinking your core temperature is lower than it really is, she adds.

Harrington, who runs courses in the workplace to help employees improve their sleep, is the author of The Sleep Diet, a new book about the impact of chronic sleeplessness on overweight.

Besides raising levels of the 'hunger' hormone ghrelin that increases appetite, chronic lack of sleep can also slow down our metabolism so that we burn fewer kilojoules and conserve more fat. But this book has such good information on getting to sleep, including a DIY sleep program, that you don't necessarily need a weight problem to benefit from it – just having a sleep problem will do.

We all know about avoiding caffeine too close to bedtime, for instance, but Harrington explains that it can take five hours or more for the caffeine in a cup of coffee – typically around 100mg - to drop down to 50mg.

"This means that if you have a cup of coffee at 6pm, you'll still have enough caffeine in your blood at 11pm to make it hard to initiate sleep," she says.

We also know we need dim lighting at night to trigger melatonin, the sleep hormone that makes us feel drowsy and which is suppressed by bright light - but we may not realise that the light from a laptop screen can be enough to interfere with melatonin production. That's why Harrington's advice to the sleep deprived is to shut your laptop at 8.30pm – and don't sneak a peek at your email before bed.

But for many of Harrington's clients, the problem isn't falling asleep, it's waking up in the early hours and becoming so wound up about getting back to sleep that anxiety keeps them awake.

"For them, one of the biggest changes comes when you explain exactly how sleep works and why waking up at night and even staying awake for up to 20 to 25 minutes is normal – just knowing this help to relieve the anxiety."

The Sleep Diet by Dr Carmel Harrington is published by Pan Macmillan Australia, RRP $24.99.